Pumps for transferring a fluid by pressure by an impeller that rotates with respect to a shaft provided inside a housing have hitherto been known (see PTL 1 and PTL 2). Some of the pumps thus configured may use an annular plain bearing for rotatably supporting the impeller with respect to the shaft. When the plain bearing is fixed to the impeller, the plain bearing can move along an axial direction (thrust direction) with respect to the shaft. Therefore, an annular restrictor (such as a washer) fixed to the shaft is sometimes used to restrict the axial movement of the plain bearing.
When the rotation speed of the impeller is high, however, the impeller may move toward the restrictor. This can increase the sliding resistance of the plain bearing against the restrictor (sliding resistance between the opposing surfaces of the plain bearing and the restrictor), which may increase the rotation torque of the impeller and lower the pump performance.